This invention relates to nuclear magnetic resonance imaging methods and systems and, more particularly, to imaging of lesions and differentiation of benign and malignant lesions.
Early detection of breast cancer is of great importance in treating the disease. The primary method currently used to detect the disease is x-ray mammography. While this imaging modality is widely used, it has limitations in sensitivity and specificity.
It has been recognized for many years that magnetic resonance imaging may be used to detect breast lesions. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0) along the z direction in a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, and this signal may be received and processed to form an image. Some reports indicate that MR (magnetic resonance) images have a 100% sensitivity in the detection of malignant breast lesions. Using contrast enhanced MR imaging methods, malignant and benign tumors that cannot be seen with x-ray mammography can be seen on MR images.
Although MR imaging is an excellent modality for detecting the presence of lesions, the ability to differentiate benign lesions from malignant lesions is also required if MRI (magnetic resonance imaging) is to become a viable adjunct to x-ray mammography in the detection and diagnosis of breast cancer. Guidelines for standardization of x-ray mammography reading have defined signs of early breast cancer to be: tumor mass, usually irregularly marginated or spiculated; small, grouped (clustered) calcifications with or without a mass; poorly defined, asymmetric breast parenchyma, especially if developed since a prior exam; and, distortion of the breast parenchymal architecture by scirrhous tumor. The distinction between benign and malignant masses is generally made by analysis of the margins, shape, density and size of the detected lesions. Generally, a benign mass such as a cyst, lymph node, and fibroadenoma has a sharply circumscribed margin, oval or round shape. Fat-containing masses that are well circumscribed are always benign. Malignancies have a "benign" appearance in less than 1-2% of the cases where they are greater than 1.0 cm in diameter. Papillary, medullary, and colloid carcinomas are more likely to be well circumscribed than the more common ductal and lobular carcinomas. The most important and specific feature of malignant masses greater than 1.0 cm in diameter in x-ray mammography is a speculated margin, which is due to the infiltrative nature of the cancer. Irregularity and indistinctness of margins are less common manifestations of this phenomenon.
By incorporating the above morphologic x-ray mammography breast lesion characteristics (excepting those for calcifications) the specificity of characterizing MR-visible breast lesions has been significantly increased. The "architectural features" which have been found to be most useful in characterizing MR-visible breast lesions include: lesion border irregularity (spiculation, 76-88%); non-uniform lesion enhancement (peripheral rim enhancement, 79-92%); and enhancement patterns which simulate ductal structures. Conversely, smooth bordered or lobulated lesions, particularly with internal septations, has been found to be predictive (97-100%) of benign disease. Likewise, non-enhancement, or negative enhancement, of the lesion was (100%) predictive of benign disease. Such morphologic assessment of breast lesions requires high spatial resolution 3D MR images acquired during the immediate post contrast-phase. Using this morphologic method of characterizing MR-visible breast lesions, an overall specificity of 74% has been obtained with a sensitivity of 96% for MR-visible breast lesions for radiologic reading.
Despite the use of both architectural features and temporal contrast enhancement to characterize MR-visible breast lesions, it has not been possible to characterize lesions as either benign or malignant with 100% certainty except through use of MR-guided biopsy techniques.